1. Field of the Invention
This invention relates to a machine tool, such as a machining center, a plano-milling machine, or the like, in which a table is moved along a linear-way. More specifically this invention relates to a machine tool in which the table itself is changed to mount and dismount a workpiece on the table.
2. Description of the Related Art
In the related art, a machine tool having a method of setting a pallet table on a pallet base is used, in which the pallet table is essentially to a table top. In such a machine, a workpiece is mounted on the pallet table outside the machine, and thereafter the pallet table is set into the machine tool to reduce the downtime of the machine tool needed for workpiece changing.
In such a machine tool, there is a reduction in the rigidity of the machine, as compared to one in which the workpiece is mounted directly on an integral table, because of the existence of separation and combination forces at the pallet table/pallet base interface. This results in a reduction of the machinability and has an adverse effect on the machining accuracy.
Further, the height of the machine is increased, because the pallet table is mounted on the pallet base. Because of this increase in height, the height of the column having the tool is also increased. Thus there is also a reduction in the rigidity of the machine. This also results in a reduction of the machinability and has an adverse effect on the machining accuracy.
Furthermore, the total weight of the pallet table and the pallet base is increased, as compared to that of a single table, so that deformation of the bed is also increased. This requires an increase in the driving power and also results in adverse effects on the machining accuracy. If these adverse effects are to be avoided, a limitation of the load is required. This also results in a reduction in the machinability or the machining specifications because of possible relative motion between the pallet table and pallet base.
In another known method for changing a workpiece of a machine tool, such as a machining center, a plurality of tables are prepared.
In this case, a workpiece is mounted and machined on one table. During the machining process for the workpiece, another workpiece is mounted on or dismounted from another table. In such case, because the workpiece is mounted on and dismounted from the other table during the machining process for another workpiece, the mechanical accuracy is influenced. Also it is difficult to increase the number of tables because a machining center is designed for only a specific number of tables and lacks flexibility for adding more.
By contrast, in a machine tool having pallet tables, it is much easier to increase the number of pallet tables. Accordingly it is desired to provide a machine tool having changeable tables, such as pallet tables, which are low in height and relatively free of adverse effects to machining accuracy.